Metallicities, Age--Metallicity relationships, and Kinematics of Red Giant Branch Stars in the Outer Disk of the Large Magellanic Cloud

TL;DR

This study investigates the outer disk of the Large Magellanic Cloud, analyzing stellar ages, metallicities, and kinematics to understand its formation history, revealing an outside-in formation pattern and disk-like kinematics for the stars.

Contribution

It provides new insights into the age-metallicity relationship and kinematic properties of outer LMC stars, supporting an outside-in formation scenario.

Findings

Metallicity decreases with radius for young stars.

Stars follow a rotational cold disk kinematics.

Oldest stars are in a thicker disk than younger ones.

Abstract

The outer disk of the LMC is studied in order to unveil clues about its formation and evolution. Complementing our previous studies in innermost fields (3<R<7 kpc), we obtained deep color magnitude diagrams in 6 fields with radius from 5.2 to 9.2 kpc. The comparison with isochrones shows that while the oldest population is approximately coeval in all fields, the age of the youngest populations increases with increasing radius. Low-resolution spectroscopy in the infrared CaII triplet region has been obtained for about 150 stars near the tip red giant branch in the same fields. Radial velocities and stellar metallicities have been obtained from these spectra. The metallicity distribution of each field has been analyzed together with those previously studied. The metal content of the most metal-poor objects, which are also the oldest according to the derived age-metallicity relationships, is similar in all fields independently of the radius. However, while the metallicity of the most metal-rich objects measured, which are the youngest ones, remains constant in the inner 6 kpc, it decreases with increasing radius from there off. The same is true for the mean metallicity. According to the derived age-metallicity relationships, which are consistent with being the same in all fields, this result may be interpreted as an outside-in formation scheme in opposition with the inside-out scenario predicted by LCDM cosmology for a galaxy like the LMC. The analysis of the radial velocities of our sample of giants shows that they follow a rotational cold disk kinematics. The velocity dispersion increases as metallicity decreases indicating that the most metal-poor/oldest objects are distributed in a thicker disk than the most metal-rich/youngest ones in agreement with the findings in other disks such as that of the Milky Way. They do not seem to be part of a hot halo, if one exists in the LMC.